An automated fiber-optic SPR platform with a self-assembled peptide interface for sPD-L1 analysis

An automated dip-probe fiber-optic surface plasmon resonance (FO-SPR) method was developed for soluble programmed death-ligand 1 (sPD-L1) quantification. The system employs a one-step self-assembled epitope-mimicking peptide (EMP) as the capture element. Through a terminal cysteine, the EMP binds directly onto the gold probe to form a capture interface, eliminating the need for chemical cross-linking. Coupled with an antibody-conjugated gold nanoparticle (Ab-AuNP) sandwich assay, the FO-SPR platform achieved a limit of detection of 20.5 ng/mL in diluted human serum with recoveries ranging from 93.7% to 106.6%. Independently prepared probes yielded intra- and inter-assay relative standard deviations below 6%, and EMP-functionalized probes retained more than 90% of their initial response after four weeks of storage at 4 °C. By pairing this peptide interface with a direct-immersion format that avoids fluidic clogging, this work delivers a practical and automated FO-SPR methodology for diverse bioanalytical applications.